Lubricant compositions are known to provide a lubricating oil film between different components within an internal combustion engine. The lubricating oil film is known to improve oxidative and thermal stability, to help reduce corrosive and adhesive wear. In addition the lubricant compositions help impart cleanliness to the engine and associated engine components (such as a 2-stroke internal combustion engine cylinder liner (and piston)). One of the important features that lubricants have that help in protecting the engine is the lubricating oil film “thickness,” i.e., viscosity. However, for a lubricant to perform at an optimum level, the oil film thickness has to provide a balance between being thin enough to spread efficiently over a surface but yet thick enough to provide a continuous film that does not readily evaporate when exposed to engine operating temperatures.
Control of lubricant thickness has been achieved by employing a wide variety of polymeric viscosity modifiers and/or bright stock. Polymeric viscosity modifiers include, for example, polymethacrylates, polyolefins and polyisobutylenes. International Publication WO 99/64543 (Carey et al., published 16 December 1999), for example, discloses diesel cylinder oil having a viscosity index of at least 95 and a total base number of at least 40 mg KOH/g. The oil further contains 1 wt % to 20 wt % of a liquid polyisobutylene having a viscosity from 1500 to 8000 mm2/s at 100° C. However, viscosity modifiers are believed to increase piston deposit levels.
Bright Stock is a lubricating oil component of high viscosity typically obtained from residues of petroleum distillation. Bright stock has been utilized in many lubricants, particularly those requiring SAE 50 (16.3 mm2/s to 21.9 mm2/s) viscosities, bright stock is believed to provide a lubricant with reduced oxidative or thermal stability. However, the availability of bright stock is diminishing, resulting in high volume uses such as those for 2-stroke or 4-stroke marine or stationary power engines requiring alternative solutions to impart the desired viscometrics in lubricants.
International publication WO 2008/021737 (Cook et al. published 21 February 2008) discloses a method of lubricating an internal combustion engine with a power output of at least 1600 kilowatts, comprising supplying to the internal combustion engine a lubricating composition comprising (a) an oil of lubricating viscosity and (b) at least 0.5 wt % of a carboxylic acid or an anhydride thereof, wherein the carboxylic acid or anhydride thereof has a hydrocarbyl group of at least about 10 carbon atoms. Also disclosed in a long list of corrosion inhibitors is the possibility to include Synalox® corrosion inhibitor. The Synalox® corrosion inhibitor is typically a homopolymer or copolymer of propylene oxide. However, there is no teaching towards the selection of the Synalox® corrosion inhibitor over other corrosion inhibitors.